Gas turbine engines include one or more rotor shafts supported by bearings which, in turn, are supported by annular frames. The frame includes an annular casing spaced radially outwardly from an annular hub, with a plurality of circumferentially spaced apart struts extending therebetween. The struts may be integrally formed with the casing and hub in a common casting, for example, or may be suitable bolted thereto. In either configuration, the overall frame must have suitable structural rigidity for supporting the rotor shaft to minimize deflections thereof during operation.
Furthermore, frames disposed downstream of the engine's combustor, are, therefore, subject to the hot combustion gases which flow downstream from the combustor and through the engine's turbine which extracts energy therefrom for rotating the shaft. Since the struts extend radially inwardly from the casing, they necessarily pass through the combustion gases and must, therefore, be suitably protected from the heat thereof. Accordingly, conventional fairings typically surround the struts for providing a barrier against the hot combustion gases, and through which fairings cooling air may be channeled for preventing elevated temperatures of the frame.
Such a frame including fairings to protect against the combustion gases, typically referred to as a turbine frame, must, of course, be configured to allow the assembly thereof. In one conventional configuration, the casing, struts, and hub are an integral cast member, and, therefore, each of the fairings must be configured for assembly around each strut. For example, the fairing may be a sheet metal structure having a radial splitline which allows the fairing to be elastically opened for assembly around a respective strut, and then the fairing is suitably joined together at its splitline to complete the assembly.
In an alternative configuration, the struts may be integrally joined at one end to ether the casing or the hub, and at its other end bolted to the complementary hub or casing. In this way, the fairing may be an integral hollow member which can be positioned over the free end of the strut prior to joining the strut free end to its respective casing or hub. In such an assembly, provisions must be provided to ensure that the joint between the strut end and the casing or hub provides suitable rigidity to ensure an overall rigid frame to suitably support the rotor shaft. In a typical conventional configuration wherein the strut outer end is bolted to the casing, the casing is an annular member having a plurality of radially extending generally inversely U-shaped slots which receive the strut ends. Conventional expansion bolts extend in generally tangential directions through the spaced apart radial legs defining the U-slot for rigidly joining the strut end to the casing. The expansion bolts provide zero clearance between where they pass through the strut end and the casing to ensure effective transmittal of both compression and tension loads between the strut and the casing. This arrangement allows assembly of the expansion bolts from the exterior of the casing.
However, the U-slots themselves provide circumferentially spaced apart discontinuities along the circumference of the casing which interrupt the hoop stress carrying capability of the casing and, therefore, decrease the overall rigidity of the frame. This reduction in rigidity may be minimized by making the strut outer end as small as possible in transverse configuration, with a practical limit being the transverse configuration of the central portion of the strut itself. This relatively small size of the strut outer end also ensures that the fairing surrounding the strut may be made as small as possible since it must be typically assembled over the strut outer end to complete the assembly of the turbine frame. Minimizing the strut, and hence, the fairing, reduces both weight and aerodynamic penalties.
Accordingly, it is desirable to have a turbine frame having reduced size struts for reducing the size of the fairing surrounding the strut while also rigidly mounting the strut to both the casing and the hub. In a configuration where the strut is bolted to either the casing or the hub, the joint therebetween should provide suitable rigidity to ensure the overall rigidity of the entire turbine frame for carrying both compression and tension loads through the struts without undesirable deflections of the hub which would affect the proper positioning of the rotor shaft supported thereby. Furthermore, it is also preferable to provide hollow struts to form a common channel through the casing and the hub for channeling air therethrough or for carrying service pipes such as lube oil or scavenge oil pipes into the engine sump located below the hub. This must be done without significantly reducing the overall structural rigidity of the turbine frame due to the required apertures, or interruptions, in either the casing or the hub for carrying the airflow or service pipes therethrough.